The present invention relates to polycrystalline diamond or cubic boron nitride (CBN) compacts made by a high pressure/high temperature process (HP/HT) and more particularly to such compacts in a sandwich configuration.
A compact is a sintered polycrystalline mass of abrasive particles (e.g. diamond) bonded together to form an integral, tough, coherent, high strength mass. A composite compact is a compact bonded to a substrate material, such as a cemented metal carbide (e.g. cobalt cemented tungsten carbide). The metal bonded carbide mass generally is selected from the group consisting of tungsten, titanium, tantalum carbides and mixtures thereof with metal bonding material therein normally being present in a quantity from about 6 to 25 weight percent and selected from the group consisting of cobalt, nickel, iron and mixtures thereof. Other metal carbides can be used.
Compacts or composite compacts may be used as blanks for cutting tools, drill bits, dressing tools, and wear parts. Compacts made in a cylindrical configuration have been used to make wire drawing dies (see U.S. Pat. No. 3,381,428).
One method for manufacturing diamond compacts involves the steps of:
A. placing within a protective shield metal enclosure which is disposed within the reaction cell of an HP/HT apparatus; PA0 B. subjecting the contents of the cell to conditions of temperature, pressure and time (typically at least 50 kbar, at least 1300.degree. C. and 3-120 minutes) sufficient to give bonding between adjacent crystal grains.
(1) a mass of diamond crystals; and PA1 (2) a mass of catalyst metal or alloy containing catalyst metal in contact with the mass of diamond crystals; and
The mass of catalyst metal could be in the form of a disc of one of the well known catalysts or an alloy containing at least one catalyst metal for diamond crystallization. Under the HP/HT condition, a wave of liquid metal advances through the dense diamond (or CBN material as described below), and the catalyst metal (in liquid form) makes itself available as a catalyst or solvent for recrystallization or crystal intergrowth. The terms catalyst and catalyst/solvent are used interchangeably. This process is sometimes known as the sweep-through method, i.e. the catalyst sweeps (or advances or diffuses) through the crystalline mass.
The relative shapes of the abrasive mass and catalyst can be varied. For example, the mass of diamond can be cylindrical, and the catalyst can be an annular shape surrounding the cylinder of abrasive crystals or a disc on top or below the diamond mass.
The source of catalyst may also be cemented metal carbide or carbide molding powder (which may be cold pressed to shape) wherein the cementing agent is a catalyst or solvent for diamond recrystallization or growth.
The catalyst is generally selected from cobalt, nickel and iron, but can be selected from any of the known catalysts which also include ruthenium, rhodium, palladium, platinum, chromium, manganese, tantalum or mixtures or alloys of catalysts. Catalyst may be mixed with the abrasive crystals in addition to or instead of being a separate mass adjacent to the abrasive crystals.
High temperature and high pressure in the diamond stable region are applied for a time sufficient to bond or sinter the diamond crystals together. The diamond stable region is the range of pressure temperature conditions under which diamond is thermodynamically stable. On a pressure-temperature phase diagram, it is the high pressure side, above the equilibrium line between diamond and graphite. The resulting compact is characterized particularly by diamond-to-diamond bonding, i.e., bonding between adjacent grains whereby there are parts of the crystal lattice which are shared between neighboring crystal grains (resulting from recrystallization at HP/HT conditions). The diamond concentration preferably is at least 70 volume percent of the diamond mass (i.e. excluding any substrate mass). Methods for making diamond compacts are detailed in U.S. Pat. Nos. 3,141,746; 3,745,623; 3,609,818; 3,831,428; and 3,850,591 (all of which are incorporated herein by reference).
Cubic boron nitride compacts are manufactured in a similar manner to that just described for diamond. However, in making a CBN compact by the sweep-through method, the metal swept through into the CBN crystal mass may or may not be a catalyst or solvent for CBN recrystallization. Thus, a mass of polycrystalline CBN can be bonded to a cobalt cemented tungsten carbide substrate by sweep through of the cobalt ingredient into the interstices of the CBN mass under HP/HT conditions, even though cobalt is not a catalyst for CBN. This interstitial cobalt binds the polycrystalline CBN to the cemented tungsten carbide substrate. Nevertheless, the term catalyst will be used to describe the bonding or sintering metal swept into a CBN particle mass for the sake of convenience. In either the case of diamond or CBN composite compacts, the cobalt depletion of the substrate often, though not necessarily, is not enough to be detrimental to the support function of the substrate.
The HP/HT sintering process for CBN is carried out in the CBN stable region which is in the range of pressure and temperature conditions under which CBN is thermodynamically stable. CBN concentration is preferably at least 70 volume percent of the CBN mass. Methods for making CBN compacts are detailed in U.S. Pat. Nos. 3,233,988; 3,743,489; and 3,767,371, which are incorporated herein by reference. Crystal intergrowth or crystal-to-crystal bonding between neighboring CBN grains (as described for diamond compacts) is believed to be present.
Re-sintered polycrystalline CBN compacts, disclosed in U.S. Pat. No. 4,673,414, are made by sintering substantially catalyst-free boron-rich polycrystalline cubic boron nitride particles in an HP/HT apparatus. Particles are subjected to a pressure and temperature adequate to re-sinter the particles, the temperature being below the CBN re-conversion temperature. The pressure typically is in excess of about 45 Kbars and the temperature ranges from at least about 1500.degree. C. to below the CBN reconversion temperature. Direct conversion CBN compacts, disclosed in U.S. Pat. No. 4,188,194 are made by placing preferentially oriented pyrolytic hexagonal boron nitride in a reaction cell. The boron nitride is substantially free of catalytically active materials and is subjected to a pressure of between about 50 and 100 Kbars and a temperature of at least 1800.degree. C., and within the cubic boron nitride stable region of the boron nitride phase diagram.
The manufacture of thermally stable compacts is described in U.S. Pat. Nos. 4,228,248 and 4,224,380, (both of which are incorporated herein by reference). These patents teach the removal of substantially all of the metallic (catalyst) phase from compacts to yield a compact comprising self-bonded diamond or CBN particles with an interconnected network of pores dispersed throughout. Such compacts can withstand exposure to temperatures of about 1200.degree. C. to 1300.degree. C. without substantial thermal degradation, an advantage over the compacts of, for example, U.S. Pat. No. 3,745,623 which are thermally degraded at a temperature of between about 700.degree. C. and 900.degree. C. Thermal degradation is indicated by a marked loss (e.g. 50%) in physical properties, such as decreased abrasion resistance, tansverse rupture strength and modulus of elasticity with increasing temperatures. The metallic or catalyst phase is removed by acid treatment, liquid zinc extraction, electrolytic depletion or similar processes. The compacts of this type will be referred to throughout as thermally stable compacts.
Fine diamond feed material has always been more difficult to sinter by the sweep-through method. Generally, sintering becomes increasingly difficult as the feed material particle size decreases. Smaller sizes of diamond feed materials (particles having a nominal largest dimension of 4-8 microns or less) have been a problem for some time because of their large surface area and small size causes more difficulties when cleaning, handling or loading the fine powder into a reaction cell. However, it is also known that as the grain size of diamond compacts decreases, transverse rupture strength increases, thus giving compacts made with smaller particles an advantage. Another advantage is the compact's finer cutting edge which may result in less workpiece damage. Under the high pressures (e.g. 50 kbar and greater) applied during the HP/HT process, such fine abrasive crystals compact resulting in a rather high packing density and a very fine pore structure. The resulting diamond mass, therefore, is dense and offers resistance to the percolation or sweep of catalyst metal through the interstices.
Sandwich compacts comprise polycrystalline diamond or CBN sintered compacts which are interposed between two support layers such as a metal or metal alloy, or a cemented carbide substrate. Sandwich compacts have been proposed in U.S. Pat. Nos. 4,219,339; 4,229,186; and 4,255,165. These citations provide a variety of sandwich compact configurations. Regardless of the configuration, such sandwich compacts are difficult to reliably fabricate on an economical basis due to an inherent fracturing problem particularly at the diamond-substrate interface. Such fracturing problem is a major mode of failure under the HP/HT fabrication process or in a later brazing process. When finer diamond feed material is used, the fabrication of sandwich compacts is further exacerbated due to the difficulty in the catalyst sweeping through the interstices from the carbide support.